Ultra High Frequency (UHF) land mobile radio applications cover a wide frequency range (for example, nominally 380 MHz to 512 MHz) and a wide corresponding percentage bandwidth (for example, as much as 30%). To reduce cost and complexity and to better serve the customer base, it is desirable to serve this wide UHF frequency range with the fewest possible different models of UHF radios. This means that each different radio model needs to cover a relatively large part of the UHF band--and thus a wide frequency range. For example, it would be desirable to develop a wideband UHF radio that could cover a full bandsplit (i.e., 403-470 MHz). Most prior high performance UHF radios have covered only about half this frequency span.
One problem in designing such wideband UHF radios is the need to minimize spurious signal content. Spurious signals are easier to eliminate in narrowband radios because narrow band radios can include highly selective narrow band filters. Such narrowband filters can be used to filter out all but the narrow range of desired radio operating frequencies. This reduces the number of spurious signals generated within the radio that can reach the antenna where they can be radiated and cause interference with other radio services and equipment. Since a wideband UHF radio must operate over a relatively wide band of frequencies, it is generally not possible to use such highly selective narrowband filters. This increases the likelihood that the radio will conduct undesirable, potentially interfering spurious signals to its antenna.
To minimize cost and size while maintaining flexibility and performance, a portable radio in the UHF band 403-470 MHz should preferably use a superheterodyne double conversion receiver and transmitter both of which are fed by a common synthesizer controlled first local oscillator and a common second local oscillator that is locked to the synthesizer. This particular architecture has advantages in terms of selectivity, sensitivity, and cost, but it has the potential problem that the second local oscillator, and/or its associated reference oscillator can generate harmonics that fall within the radio's wide band operating range.
Lowering spurious output signals is important not only for good engineering, but also to meet strict regulatory requirements imposed by various governments. In the United States, regulations of the Federal Communications Commission forbid radios from emitting more than a certain maximum amount of spurious signal content. In Europe, where many UHF radios are deployed, regulations such as ETSI are especially stringent in their requirements to minimize conducted spurious signal output. Meeting these low spurious signal output requirements presents a significant challenge to the radio designer.
We have discovered a new and improved architecture and frequency plan for a wideband UHF radio that minimizes spurious output signal emissions.
In accordance with one aspect provided by this invention, we use a low value for the ratio of the maximum RF operating frequency value to the (first) intermediate frequency (IF) used by the radio as part of its internal superheterodyne conversion process. We have discovered that if the minimum value or a value sufficiently close to the minimum value of this ratio is used, then the radio design can be optimized based on minimizing spurious outputs and the effects of inherently present major harmonics.
In more detail, we have discovered that using a relatively high second local oscillator frequency (e.g., .sup..about. 125 MHz) allows us to place the inherent third and fourth harmonics of this local oscillator frequency (e.g., 375 MHz and 500 MHz, respectively) on either side of the desired UHF radio coverage band (e.g., 403 MHz to 470 MHz)--leaving sufficient frequency spacing between these harmonics and the radio passband such that wideband transmitter output frequency filters can suppress the harmonics to acceptably low levels.
The second local oscillator is not the only part of the radio that can cause spurious outputs. In accordance with another aspect provided by the present invention, we have determined that the reference oscillator used to lock the synthesizers often generates spurious signals which can cause major problems in the radio. For example, such spurious signals can interfere with received signals and can also induce the transmitter to radiate and/or transmit unwanted signals. We have found that this is particularly true if a harmonic of the reference frequency is reinforced by being close to the second local oscillator frequency and/or close to a harmonic of the second local oscillator frequency (see, e.g., FIG. 2). We have discovered that we can optimize the second local oscillator frequency to minimize such spurious outputs that may be caused by harmonics of the reference oscillator mixing with harmonics of the second local oscillator.
In accordance with this aspect of the invention, we have discovered that if we carefully optimize the second local oscillator frequency relative to the reference frequency, we can be assured as to where the worst harmonic combination of these two signal contents will occur. In accordance with one preferred example embodiment of our invention, we have placed the second local oscillator exactly between the harmonics of the reference oscillator. This allows certain higher order harmonics of the reference oscillator to exactly coincide with certain harmonics of the second local oscillator while ensuring that none of the reference oscillator's lower order harmonics line up with the second local oscillator harmonics--so that all reference oscillator harmonics of significant amplitude will also be outside of the radio operating band and so that the potential of harmful products resulting from reference oscillator harmonics mixing with second local oscillator harmonics will also be minimized. While this technique does not, of course, eliminate all reference oscillator harmonics within the radio passband, it ensures that only higher order harmonics (which are lower in amplitude and thus inherently well suppressed) fall within the passband and also reduces the number of mixer products that fall within that passband.
In accordance with yet another aspect provided by this invention, the relatively high second local oscillator frequency (which is used for both transmit and receive) provides a relatively high (e.g., .sup..about. 125 MHz) receiver first intermediate frequency (IF). First IF filtering must be highly selective such that when used with the appropriate 2.sup.nd IF filters, the combination eliminates adjacent channel interference and intermodulation from other channels just 12.5 to 25 kHz away from a selected operating frequency. We have discovered that a fundamental monolithic crystal filter can be used to provide intermediate frequency filtering at such high intermediate frequencies to meet selectivity, insertion loss, and intermodulation requirements. In accordance with this aspect of the invention, a fundamental crystal IF filter can be constructed based on a four-pole design in two ceramic flat packs. This type of IF crystal filter uniquely addresses the size, cost, ease of integration and manufacturability constraints that are imposed by portable radio design.
In accordance with a further aspect provided by the present invention, we have discovered that we can generate a relatively high (e.g., .sup..about. 125 MHz) second local oscillator signal with extremely low phase noise by using a fundamental mode crystal such as a high frequency inverted mesa crystal. The lowest noise performance is achieved by a fundamental mode crystal as opposed to an overtone oscillator crystal because of the fundamental mode crystal's lower resistance. Furthermore, the fundamental crystal has other advantages such as better trim range. In accordance with this aspect of the present invention, we have provided a fundamental mode inverted mesa oscillator crystal for production that can generate a relatively high second local oscillator signal (e.g., .sup..about. 125 MHz) which produces exceptionally low phase noise.
The following is a non-exhaustive list of additional features and/or advantages provided by example preferred embodiments of present invention:
Novel process for optimizing the frequency plan of a UHF portable radio resulting in significant advantages for making a realizable radio in a cost effective manner for the array of specifications and requirements that must be satisfied; PA1 New advantageous design for a wideband 403-470 MHz UHF portable radio; PA1 A process for meeting stringent European (ETSI) spurious emission requirements to minimize spurious transmitter output levels and receiver spurs; PA1 Advantageous novel use of a high frequency crystal filter and high frequency crystal oscillator; PA1 Novel procedure for minimizing major synthesizer spurious signals which would fall in the RF band--thereby eliminating the need for additional circuitry to attenuate these spurious signals; PA1 Eases the requirements on phase noise of the first synthesizer/local oscillator--reducing radio complexity and cost; PA1 Provides a procedure for determining whether the front end RF image filter can be realized using low cost small standard LC elements (e.g., ceramic chip inductors and capacitors) instead of requiring more expensive SAW and/or ceramic filters; PA1 Harmonics of the second local oscillator frequency can be optimally placed out of the RF band where they can be conveniently filtered, and in-band products are of a high order and thus are inherently well-suppressed; PA1 High first local oscillator frequency places the first image sufficiently far away from the RF passband to allow use of lumped element filters in the RF section (such lumped element filters are low cost, small in size, and provide optimal bandwidth for a wideband UHF radio); PA1 Minimizes the major spurious outputs caused by the harmonics from the synthesizer and the vctcxo reference oscillator, thereby simplifying the receiver and transmitter filtering. PA1 A receive IF fundamental monolithic crystal filter with low insertion loss designed and built at .sup..about. 125 MHz to satisfy selectivity, insertion loss and intermodulation requirements; PA1 A second LO for receive which is common to transmit and which can achieve extremely low phase noise by using a fundamental mode crystal; PA1 The phase noise of the second local oscillator and the high selectivity of the first and second receive IF filters enable nearly all adjacent channel and intermodulation rejection requirements--meaning less stringent requirements--to be allocated to the phase noise performance of the first local oscillator (because the overall phase noise results from a combination of the phase noise of the first local oscillator phase noise and the second local oscillator phase noise, we can tolerate much higher first local oscillator phase noise--in order for example to achieve fast synthesizer settling time at lower cost--and still achieve acceptably low overall phase noise performance because of the 2.sup.nd local oscillator's extremely low phase noise); PA1 Frequency plan places the 403-470 MHz transmit band optimally between the third and fourth harmonics of a 124.8 MHz transmit local oscillator frequency so that these harmonic spurs can be filtered effectively; PA1 In-band transmit spurious levels are minimized, being higher-order (thus lower amplitude) spurs generated in the transmit modulator; PA1 The receiver first IF frequency of 125.25 MHz pushes the first image band out to 653.5-720.5 MHz, enabling image rejection filtering with tunable lumped element filtering instead of the usual bulky ceramic or helical image filters; PA1 A fundamental mode 125.25 MHz 4-pole crystal is used for second image and intermodulation filtering while integrating easily with surrounding IF circuitry; PA1 The second local oscillator frequency is fixed at 124.8 MHz and is stabilized by a fundamental 124.8 MHz crystal resonator, which gives extremely low second local oscillator phase noise; PA1 To enable cost-effective dual mode operation in 25 kHz and 12.5 kHz channel-spaced systems, final selectivity filtering at the second IF can be comprised of a permanently connected cascade of wider band filters for 25 kHz channelization, and a narrow band ceramic filter switched in for 12.5 kHz channelization; PA1 The phase noise and selectivity performance permit nearly all the adjacent channel and intermodulation degradation to be allocated to the phase noise performance of the first local oscillator, where it is most advantageous to compromise phase noise performance; PA1 The frequency plan uses an unusually low RF-to-first-IF frequency ratio of 3.75 compared with typical 800-900 MHz radios with ratios of 10 or so; PA1 High specification operation using available, manufacturable, off-the-shelf components; PA1 Can use the same chassis and board footprint as higher frequency (e.g., 800-900 MHz) portables; PA1 Alternate embodiment can use, before the first mixer for image rejection, a tracking filter which passes particular RF frequency being used and attenuates the half IF frequency and image frequency to maintain unusually low and constant insertion loss over the entire tracking range.